Gas detectors are utilized in various fields, such as in the fields of medical treatments, environmental measurements and various kinds of testing. Gas detectors are utilized, for example, to detect exhaust gas of air conditioners, exhaust gas of automobiles, exhaust gas from factories, leaking gas in domestic households, toxic gases such as gases associated with sick house syndrome, and odorous gas of raw garbage and pets and the like.
Examples of gas detection methods include semiconductor type detection, contact combustion type detection, thermal conductivity type detection, and NDIR (non-dispersive infrared) type detection. The semiconductor type detection detects changes in electrical resistance caused by gas that is exposed to a semiconductor, and although this type of detection offers the advantages of being compact and having a high degree of mass productivity, there is the drawback that the semiconductor type detection lacks stability. The contact combustion type detection combusts gas inside an apparatus, and is limited to combustible gases. The thermal conductivity type detection detects a temperature change that is in accordance with a difference in the thermal conductivity of a gas, and has the drawback of low sensitivity. The NDIR type detection has an advantage of being applicable to various kinds of gases, having high sensitivity and providing long-term operational stability. However, there is a drawback that the apparatus size may be larger as sufficient optical path length is required.
Patent Literature 1 discloses, in its abstract, that, in order to keep the intensity of output light from an infrared light source 8 constant, an infrared detector 9B detects infrared light passing through an optical filter 10 B and having a wavelength that is not absorbed by gas, and controls an applied voltage to the infrared light source 8.